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Chapter 25
Domain Name System
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Domain Name System
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What is DNS?
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Internet Directory Service
A client-server application that maps host names into their
corresponding IP addresses
Mapping host names into their corresponding IP addresses
is called name resolution or name translation or name
mapping or Address Resolution
Why we need to use names instead of IP
numbers?
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IP addresses are difficult to remember
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Problem: Network only understands numeric
addresses
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Solution:
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Use alphanumeric names to refer to hosts
Add a distributed, hierarchical protocol (called DNS) to
map between alphanumeric host names and IP addresses
Name Space
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IP addresses are unique Host names must be unique
How to manage this large number of names?
Solution:
Each name is made of several parts (hierarchical)
Each part is called a label
Names are defined on tree structure with the root at the top
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Each node has a label
DNS requires that children of a node (nodes that branch from the same
node) have different labels to guarantee uniqueness
This will allow the control of names assignment to be decentralized
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This is called hierarchical name space
A central authority IANA assigns the part of the name that defines the
nature of the organization (com, net, SA, CA, …) and its name (IEEE,
CNN, McGraw, Intel,…)
The rest of the name is managed by the local organization (CIS, Eng,
sales, …)
The DNS Name Space
The Internet is divided into more than 200 top-level domains
Domain: It is subtree of the domain name space and consists of group of hosts that are under
the administrative control of a single entity such as a company or a government agency.
Each domain is subdivided into subdomains
The leaves represent domains that have no subdomains
A leaf domain may contain a single host, or represent a company with thousands of hosts
Top level domains
SA
A portion of the Internet domain name space.
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Table 25.1 Generic domain labels
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Figure 25.5
Domains
 Domain is a subtree of domain name space
Domain is divided into sub-domains
 Domain name is the domain name of the node at the top of the
subtree
Sub-
SubMcGraw-Hill
Sub-
Figure 25.2
Domain names and labels
Each tree node has a domain name
Domain name is a sequence of labels
separated by (.)
Labels are read from the node up to the
root
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Figure 25.4
FQDN and PQDN
FQDN= Fully Qualified Domain Name
PQDN= Partially Qualified Domain Name (name to be
completed)
Only FQDN can be translated (Mapped into its IP address)
PQDN are used to refer to a name that belongs to the
same site. Remaining part (suffix) of the name is
completed by the DNS client program (resolver)
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Distribution of Name Spaces
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The information contained in the domain
name space must be stored.
Where ?
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Centralized?  inefficient & unreliable why?
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DNS is a distributed database system
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Heavy traffic because of requests from all over the world
Failure makes data not available
Hard to maintain
Uses a large number of computers called name servers
Organized in a hierarchical way and distributed all over
the world
No single host has all the exact mappings for all the
hosts in the Internet
Figure 25.6
Hierarchy of name servers
Knows about all
names below it
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Knows about all
names below it
Figure 25.7 Zones and domains
Zone is part of the tree that a DNS
server is responsible for or has
authority over (it either knows the
mapping or the ADDRESS OF THE
dns server that can do the mapping)
When DNS server Keeps all
information for every node under
that domain then domain and zone
are equal (in the figure, mhhe is a
domain)
COM DNS delegates some
of its responsibilities to
mhhe DNS
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When DNS server delegates part of
its authority to other servers below it
then domain and zone are not the
same thing (in the figure, com is a
zone)
Note
A primary server loads all information
from the disk file; the secondary server
loads all information from
the primary server.
When the secondary downloads
information from the primary, it is called
zone transfer.
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Root servers
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Root server: a server which has references for the top
level domains
13 root servers distributed all over the world
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Name resolution
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DNS is a client-server application
Client part is called name resolver
Server part is called Name server – Its host name starts by DNS label
Steps of resolver operations:
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A resolver receives a request from application layer (for example http)
in the form of a system call, and
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Returns the desired information in a form compatible with the local
host’s data formats.
Two techniques to map the host name to its IP address
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1- Recursive resolution :
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Host A queries the site local name server B
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If B has the answer in its cache or in its database, it returns it to A,
otherwise it send requests to one of the root name servers
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The root name server (if it does not have the answer in its cache or
in its database) sends request to another name server in the next
level of the tree say ( C )
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The process is repeated until an answer is obtained
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Then, the answer will be transmitted back until it finally reaches
the requesting host (A)
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See the next example
1- Recursive resolution (mapping) - Example
* Suppose that computer Fakhrylab.CIS.EDU.SA wants to
contact sales.mcgraw.com
* SEE NEXT SLIDE for Explanation
Local DNS for PSU
2
3
7
6
DNS.PSU.EDU.SA
5
1
8
8
DNS.com
4
Local DNS for McGraw
company
Fakhrylab.CIS.PSU.EDU.SA
DNS.com
DNS.mcgraw.com
** Only Nodes that can do
mapping are shown
** DNS servers have DNS label
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Books.mcgraw.com
Sales.mcgraw.com
1- Recursive resolution – Example (cont’d.)
In the previous example, the mapping will be done as follows:
1- Host Fakhrylab.CIS.PSU.EDU.SA contacts the local name server
(DNS.PSU.EDU.SA) to query for the IP address of host sales.McGraw.com
2- If (DNS.PSU.EDU.SA) does not have the answer in its cache or in its database, it will
contact the root name server to query for the IP address of host sales.McGraw.com
3- If the root name server does not have the answer in its cache or in its database, it will
contact the name server responsible for the .com domain (DNS.com ) to query for the IP
address of host sales.McGraw.com
4- If (DNS.com) does not have the answer in its cache or in its database, it will contact
(DNS.McGraw.com) which has the IP address for host (Sales.McGraw.com)
5- (DNS.McGraw.com) will return the answer to (DNS.com)
6- (DNS.com) will return the answer to the root name server
7- The root name server will return the answer to (DNS.PSU.EDU.SA)
8- DNS.PSU.EDU.SA will return the answer to Host Fakhrylab.CIS.PSU.EDU.SA
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2- Iterative resolution (mapping)
 Host A sends a query to local name server B
 If B has the answer in its cache or in its database, it returns it to A, otherwise it sends a
reply to A that contains the IP address of the root name server
 Host A then sends a query directly to the root name server
 The root name server (if it does not have the answer in its cache or in its database)
sends a reply to A that contains the IP address of name server in the next level of the
tree say ( C )
 Host A then sends a query directly to name server C
 The process is repeated until A received the IP address for the requested mapping
 SEE the next example
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2- Iterative resolution - Example
Suppose that computer Fakhrylab.CIS.EDU.SA wants to contact
sales.mcgraw.com
** DNS servers have DNS
label
* SEE NEXT SLIDE for Explanation
3
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DNS.com
DNS.PSU.EDU.SA
1
5
2
6
Fakhrylab.CIS.PSU.EDU.SA
DNS.mcgraw.com
7
8
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Books.mcgraw.com Sales.mcgraw.com
2- Iterative resolution – Example (cont’d.)
In the previous example, the mapping will be done as follows:
1- Host Fakhrylab.CIS.PSU>EDU.SA contacts the local name server (DNS.PSU.EDU.SA) to
query for the IP address of host sales.McGraw.com
2- If (DNS.PSU.EDU.SA) does not have the answer in its cache or in its database, it will
reply to host Fakhrylab.CIS.EDU.SA with the IP address of the root name server
3- Host Fakhrylab.CIS.EDU.SA will contact the root name server to query for the IP
address of host sales.McGraw.com
4- If the root name server does not have the answer in its cache or in its database, it will
reply to host Fakhrylab.CIS.EDU.SA with the IP address of the name server for the
(.com) domain which is (DNS.com)
5- Host Fakhrylab.CIS.EDU.SA will contact the name server (DNS.com) to query for the IP
address of host sales.McGraw.com
6- If (DNS.com) does not have the answer in its cache or in its database, it will reply to
host Fakhrylab.CIS.EDU.SA with the IP address of the name server DNS.McGraw.com
which is the local name server for domain McGraw.com
7- Host Fakhrylab.CIS.EDU.SA will contact the name server (DNS.McGraw.com) to query
for the IP address of host sales.McGraw.com
8- Since name server DNS.McGraw.com is the local name server for McGraw.com domain
it will reply to host Fakhrylab.CIS.PSU.EDU.SA with the IP address for host
sales.McGraw.com
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Note
DNS can use the services of UDP or TCP
using the well-known port 53.
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